6.0 Costs and Benefits of Storm Water BMPs

6.0

Costs and Benefits of Storm Water BMPs

Storm water best management practices (BMPs) are the primary tool to improve the

quality of urban streams and meet the requirements of NPDES permits. They include both the

structural and non-structural options reviewed in Section 5.2 of this report. Some BMPs can

represent a significant cost to communities, but these costs should be weighed against the various

benefits they provide. This chapter will focus on reviewing available data on the costs and

potential benefits of both structural and non-structural BMPs designed to improve the quality of

urban and urbanizing streams, and the larger water bodies to which they drain.

As described in previous chapters, storm water runoff can contribute loadings of nutrients,

metals, oil and grease, and litter that result in impairment of local water bodies. The extent to

which these impairments are eliminated by BMPs will depend on a number of factors, including

the number, intensity, and duration of wet weather events; BMP construction and maintenance

activities; and the site-specific water quality and physical conditions. Because these factors will

vary substantially from site to site, data and information are not available with which to develop

dollar estimates of costs and benefits for individual types of BMPs. However, EPA¡¯s national

estimates of costs and benefits associated with implementation of the NPDES Phase II rule are

discussed in Section 6.4.

6.1

Structural BMP Costs

The term structural BMPs, often referred to as ¡°Treatment BMPs,¡± refers to physical

structures designed to remove pollutants from storm water runoff, reduce downstream erosion,

provide flood control and promote groundwater recharge. In contrast with non-structural BMPs,

structural measures include some engineering design and construction.

Structural BMPs evaluated in this report include:

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Retention Basins

Detention Basins

Constructed Wetlands

Infiltration Practices

Filters

Bioretention

Biofilters (swales and filter strips).

The two infiltration systems focused on in this report are infiltration trenches and

infiltration basins. Although bioretention can serve as a filtering system or infiltration practice, it

is discussed separately because it has separate cost data and design criteria. In this report, wet

swales are assumed to have the same cost as biofilters, because there are little cost data available

on this practice. Additional information about these structural BMPs, including descriptions,

applicability and performance data can be found in Chapter 5 of this report. Other BMPs include

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experimental and proprietary products, as well as some conventional structures such as water

quality inlets. They are not included in this analysis because sufficient data are not available to

support either the performance or the cost of these practices.

6.1.1 Base Capital Costs

The base capital costs refer primarily to the cost of constructing the BMP. This may

include the cost of erosion and sediment control during construction. The costs of design,

geotechnical testing, legal fees, land costs, and other unexpected or additional costs are not

included in this estimate. The cost of constructing any BMP is variable and depends largely on

site conditions and drainage area. For example, if a BMP is constructed in very rocky soils, the

increased excavation costs may substantially increase the cost of construction. Also, land

acquisition costs vary greatly from site to site.4 In addition, designs vary slightly among BMP

types. A wet pond may be designed with or without various levels of landscaping, for example.

The data in Table 6-1 represent typical unit costs (dollars per cubic foot of treated water volume)

from various studies, and should be considered planning level. In the case of retention and

detention basins, ranges are used to reflect the economies of scale involved in designing these

BMPs.

4

Land cost is the largest variable influencing overall BMP cost.

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Table 6-1. Typical Base Capital Construction Costs for BMPs

BMP

Type

Typical

Cost*

($/cf)

Notes

Source

0.50-1.00

Cost range reflects economies of scale in designing

this BMP. The lowest unit cost represents approx.

150,000 cubic feet of storage, while the highest is

approx. 15,000 cubic feet. Typically, dry detention

basins are the least expensive design options among

retention and detention practices.

Adapted from

Brown and

Schueler (1997b)

Constructed

Wetland

0.60-1.25

Although little data are available to assess the cost of

wetlands, it is assumed that they are approx. 25%

more expensive (because of plant selection and

sediment forebay requirements) than retention

basins..

Adapted from

Brown and

Schueler (1997b)

Infiltration

Trench

4.00

Represents typical costs for a 100-foot long trench.

Adapted from

SWRPC (1991)

Infiltration

Basin

1.30

Represents typical costs for a 0.25-acre infiltration

basin.

Adapted from

SWRPC (1991)

Adapted from

Brown and

Schueler (1997b)

Retention and

Detention

Basins

Sand Filter

3.00-6.00

The range in costs for sand filter construction is

largely due to the different sand filter designs. Of the

three most common options available, perimeter sand

filters are moderate cost whereas surface sand filters

and underground sand filters are the most expensive.

Bioretention

5.30

Bioretention is relatively constant in cost, because it

is usually designed as a constant fraction of the total

drainage area.

Adapted from

Brown and

Schueler (1997b)

Grass

Swale

0.50

Based on cost per square foot, and assuming 6 inches

of storage in the filter.

Adapted from

SWRPC (1991)

0.00-1.30

Based on cost per square foot, and assuming 6 inches

of storage in the filter strip. The lowest cost assumes

that the buffer uses existing vegetation, and the

highest cost assumes that sod was used to establish

the filter strip.

Adapted from

SWRPC (1991)

Filter Strip

* Base year for all cost data: 1997

In some ways there is no such value as the ¡°average¡± construction cost for some BMPs,

because many BMPs can be designed for widely varying drainage areas. However, there is some

6-3

value in assessing the cost of a typical application of each BMP. The data in Table 6-2 reflect

base capital costs for typical applications of each category of BMP. It is important to note that,

since many BMPs have economies of scale, it is not practical to extrapolate these values to larger

or smaller drainage areas in many cases.

Table 6-2. Base Costs of Typical Applications of Storm Water BMPs1

Typical Cost

($/BMP)

Application

Retention

Basin

$100,000

50-Acre Residential Site

(Impervious Cover =

35%)

Adapted from Brown and

Schueler (1997b)

Wetland

$125,000

50-Acre Residential Site

(Impervious Cover =

35%)

Adapted from Brown and

Schueler (1997b)

Infiltration

Trench

$45,000

5-Acre Commercial Site

(Impervious Cover =

65%)

Adapted from SWRPC

(1991)

Infiltration

Basin

$15,000

5-Acre Commercial Site

(Impervious Cover =

65%)

Adapted from SWRPC

(1991)

Sand Filter

$35,000$70,0002,3

5-Acre Commercial Site

(Impervious Cover =

65%)

Adapted from Brown and

Schueler (1997b)

$60,000

5-Acre Commercial Site

(Impervious Cover =

65%)

Adapted from Brown and

Schueler (1997b)

$3,500

5-Acre Residential Site

(Impervious Cover =

35%)

Adapted from SWRPC

(1991)

5-Acre Residential Site

(Impervious Cover =

35%)

Adapted from SWRPC

(1991)

BMP Type

Bioretention

Grass Swale

Filter Strip

$0-$9,000

3

Data Source

1. Base costs do not include land costs.

2. Total capital costs can typically be determined by increasing these costs by approximately 30%.

3. A range is given to account for design variations.

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Although various manuals report construction cost estimates for storm water ponds, EPA

has identified only three studies that have systematically evaluated the construction costs

associated with structural BMPs since 1985. The three studies used slightly different estimation

procedures. Two of these studies were conducted in the Washington, DC region and used a

similar methodology (Wiegand et al, 1986; Brown and Schueler, 1997b). In both studies, the

costs were determined based on engineering estimates of construction costs from actual BMPs

throughout the region. In the third study, conducted in Southeastern Wisconsin, costs were

determined using standardized cost data for different elements of the BMP, and assumptions of

BMP design (SWRPC, 1991).

Any costs reported in the literature need to be adjusted for inflation and regional

differences. All costs reported in this report assume a 3 percent annual inflation rate. In addition,

studies are adjusted to the ¡°twenty cities average¡± construction cost index, to adjust for regional

biases, based on a methodology followed by the American Public Works Association (APWA,

1992). Using EPA¡¯s rainfall zones (see Figure 6-1), a cost adjustment factor is assigned to each

zone (Table 6-3). For example, rainfall region 1 has a factor of 1.12. Thus, all studies in the

Northeastern United States are divided by 1.12 in order to adjust for this bias.

Table 6-3. Regional Cost Adjustment Factors

Rainfall Zone

1

2

3

4

5

6

7

8

9

Adjustment

Factor

1.12

0.90

0.67

0.92

0.67

1.24

1.04

1.04

0.76

Source: Modified from APWA, 1992

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